topic Re: Spline drawing issue in Fusion Support Forum

Spline drawing issue

chrisdc85 — Sun, 18 May 2025 19:31:59 GMT

Hi. I was recreating a part in a new document and I noticed that the values I was copying over for a spline wasn't recreating the curve very well. I tried to re-make the spline in the original file (original drawing even!) and despite copying all the dimensions I couldn't get them to match. I think the original had been created before the last update. Is there some option I'm missing?

I've opened/closed Fusion multiple times now. If I delete and then plug the values back into the original spline I get the original shape. Any new splines I create have this new behaviour.

If this is a bug then it seems like a critical thing to fix.

Re: Spline drawing issue

TheCADWhisperer — Sun, 18 May 2025 19:58:54 GMT

@chrisdc85

Can you File>Export your *.f3d file to your local drive and then Attach it here to a Reply?

Someone will demonstrate the differences between the curves.

Re: Spline drawing issue

chrisdc85 — Sun, 18 May 2025 20:26:11 GMT

Unfortunately this is part of a confidential file so I can't share it. Is there some difference in splines beyond fit and control point versions? I've never seen such a difference and I've been using Fusion nearly from it's first launch.

Re: Spline drawing issue

TheCADWhisperer — Sun, 18 May 2025 20:31:59 GMT

@chrisdc85

You should be able to reproduce a curve in a new non-proprietary sketch.

First thing I noticed was that your sketches were not fully defined.

First thing I would do is click and drag the unconstrained geometry and observe.

Re: Spline drawing issue

chrisdc85 — Sun, 18 May 2025 20:42:36 GMT

I can recreate the new curves fine in a sketch. That behaviour I've tested. The problem is despite having the same values as the original(old spline) they are very different.

I've constrained the curves and observed the same behaviour.

The screenshot just happened to be when the original horizontal handle's angle wasn't constrained and the vertical movement of the 2 new sketches weren't constrained. I was testing out how they behaved.

It seems as if the original spline is working off a different mathematical equation to the new ones.

Re: Spline drawing issue

TheCADWhisperer — Sun, 18 May 2025 20:54:58 GMT

@chrisdc85

How old was the original file?

I vaguely recall perhaps a change in Spline calculations.

Drag the handles - especially this one at the top.

Re: Spline drawing issue

chrisdc85 — Sun, 18 May 2025 21:09:37 GMT

The very original version of this spline goes back 2 months, so before the March 24 update. I'd updated various dimensions of it since then as part of the design iterations but don't recall redrawing it.

I tested redrawing the spline on the very first iteration and the difference is smaller, but also still visible.

Do you remember when they said they made a change to the spline calculations?

I'll try to find an earlier version.

If possible can others test if new splines are the same as older splines in their drawings? Would be good to know if this is a general issue or not.

Re: Spline drawing issue

jeff_strater — Sun, 18 May 2025 23:24:48 GMT

I am not aware of any change in spline computation. That does not mean there wasn't a change, but I just don't remember hearing about one. What I suspect you are running into is a peculiarity of the Fit Point Spline - for reasons I've never understood, it has always been impossible to reproduce the same fit point spline curve, even with the same dimensions and handle dimensions/constraints. The math guys have explained it to me, and I've never quite understood it. The answer is to use CV splines, which tend to behave much more predictably.

I thought that there was a way to convert from a Fit Point Spline to a CV Spline, but I guess I must have dreamed that, because I cannot find it. So, unfortunately, you will have to re-create your curves as CV splines, but then you should get much better results.

Re: Spline drawing issue

TheCADWhisperer — Sun, 18 May 2025 23:28:39 GMT

@chrisdc85 wrote:

Do you remember when they said they made a change to the spline calculations?

Just a very very vague recolection from many many years ago. I think if I am even remembering correctly it might have been Inventor rather than Fusion.